Security review fixes: admin permissions, guest sessions, payment edge cases, assistant leaks

- Admin: refund/cancel/mark-resolved/Bazaar-link actions need change permission; question and
  curriculum import need add/delete; content browser needs view_question; guest device ids are
  shown to superusers only.
- Accounts: registering a guest's phone revokes the guest's earlier sessions; self-deletion keeps
  Redemption, UserOffer and the phone's OTP rows; the public demo guest cannot be deleted
  (DEMO_GUEST_DEVICE_IDS); display-name contact filter covers any separator and social handles.
- Payment: callback without Authority is 400; a second Bazaar purchase on a paid order is 409;
  orders snapshot credits/hearts/days at creation (payment.0011).
- Assistant: a broken diagram no longer makes a streamed answer free; a late off-topic marker
  refuses the answer (not stored, not cached); a disconnect saves the partial answer; only
  KNOWN_COLLECTIONS are searched.
- Infra: secret-key file is created atomically (workers no longer split keys); no default
  INTERNAL_SERVICE_KEY; Kavenegar key and Bazaar token kept out of error logs; Swagger/ReDoc
  pinned; schema throttled; docker compose bound to 127.0.0.1; non-ASCII digit ids are 400;
  ticket text capped at 5000.
- docs/security-before-production.md: remaining open items 24-31 and the guard tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

content/views.py

@@ -741,7 +741,7 @@ class NodeProgressListView(APIView):
         qs = NodeProgress.objects.filter(user_id=request.user.id).select_related('node')
         unit_id = request.query_params.get('unit')
         if unit_id:
-            if not unit_id.isdigit():
+            if not (unit_id.isascii() and unit_id.isdigit()):
                 raise ValidationError({'unit': 'شناسه‌ی فصل باید عدد باشد.'})
             qs = qs.filter(node__unit_id=int(unit_id))